1. Field of the Invention
The present invention relates to the field of semiconductor manufacturing, and more specifically to a method and apparatus for uniformly depositing a thin film on a substrate.
2. Discussion of Related Art
Semiconductor devices are made up of literally millions of discreet devices which are interconnected together to form functional circuits, such as microprocessors, memories and programmable logic devices. In order to improve circuit performance, the fabrication processes generally utilize low resistance metal films such as suicides to form low resistance gate electrodes, contact regions, capacitor electrodes and interconnection lines. Silicide films, such as titanium silicide, can be formed by chemical vapor deposition (CVD) processes.
FIG. 1 is an illustration of a current CVD apparatus which can be used to deposit a titanium silicide film on a semiconductor wafer or substrate. A susceptor 120 divides a chamber 112 into one portion which is below the susceptor 124 and a second portion which is above the susceptor 122. The susceptor 120 is generally mounted on a shaft 126 which rotates the susceptor about its center to achieve a more uniform processing of the wafer. A flow of processing gas such as deposition gas 115 is provided into the upper portion 122 of the chamber. The chamber generally has a gas inlet 178 at one side thereof and a gas exhaust passage 116 at the opposite side to achieve a flow of processing gas across the wafer. The susceptor 120 is heated in order to heat the wafer to a desired deposition temperature. One method to heat the susceptor is by the use of lamps 134 provided around the chamber and directing the light into the chamber and onto the susceptor 120.
In a method of depositing titanium silicide on a wafer, a deposition gas 115 containing titanium chloride (TiCl.sub.4) and a silicon source gas, such as SiH.sub.4, is fed into the reaction chamber 112. As deposition gas is fed into the chamber, it is heated by preheat ring 128 and susceptor 120 and reacts to form titanium silicide. Titanium silicide will only form or deposit on those areas of a wafer (or chamber) which will react with titanium silicide under deposition conditions. That is, titanium silicide will only form on silicon areas of the wafer such as doped and undoped polycrystalline and amorphous silicon, the silicon substrate, and silicide layers, etc. and will not form or deposit on areas which are not silicon such as insulated layers including silicon dioxide (SiO.sub.2) and silicon nitride (Si.sub.3 N.sub.4) and susceptor 120 which is generally made of silicon carbide.
A problem with depositing titanium silicide by the process and apparatus described above is that the film deposition rate is non uniform across the surface of the wafer. The first silicon containing areas 131 (i.e., reaction areas) encountered by the deposition gas 115 experience a significantly higher deposition rate than do subsequent reaction areas. As such, as illustrated in FIG. 1b, much more titanium silicide is deposited onto the edge 131 of the wafer adjacent to the gas inlet than is deposited onto the center of the wafer or onto the opposite side of the wafer. The enhanced deposition rate associated with initial reaction areas is sometimes referred to as "edge effects". Edge effects substantially reduce film thickness uniformity and can make a process entirely unmanufacturable.
Thus, what is desired is a method and apparatus for reducing edge effects in a deposition process.